Vehicle transmissions have been provided in recent years with an increasing number of individually selectable speed ratios. Seven or more speed ratios may be provided, and by doing so many factors relating to vehicle performance and economy can be enhanced.
Control strategies for automatic transmission are highly complex, taking into account numerous external and internal factors to determine speed ratio selection and the timing of a speed ratio change. These strategies are generally incorporated within a shift map which comprises a decision matrix for all circumstances of use of the transmission. The shift map is generally stored in an electronic control unit having a processor to determine which speed ratio should be selected, having regard to many parameters.
Where a large number of individually selectable speed ratios are provided in a compact transmission housing, the arrangement of components within the transmission is also highly complex, and can result in some distinctive mechanical characteristics. For example upon downshifting sequentially through the speed ratios, certain downshifts may require an increase in speed of comparatively large rotating masses. Such speed increases may be transient, until the next sequential speed ratio is selected, but may nevertheless result in disturbances which are noticeable to vehicle occupants.
It would be desirable to eliminate such disturbances without changing fundamentally the internal arrangement of transmission components, particularly since a particular transmission may be used in several vehicle variants, and only some variants may exhibit disturbances which require to be eliminated.
The control strategy of an automatic transmission generally requires successive engagement of speed ratios in numerical sequence, in ascending or descending order. By this means the vehicle can provide maximum acceleration when required, and the control sequence of the transmission is somewhat simplified. However it will be understood that each change of speed ratio generally requires the movement of some transmission components and accordingly has a shift time associated therewith. In certain circumstances, for example during severe braking, the speed ratio selected by the transmission during downshifting may lag the ideal speed ratio.
It would be desirable for the selected speed ratio to match the ideal speed ratio more closely and/or quickly, so that for example a demand for acceleration is not delayed whilst the transmission completes a downshift sequence.
Many automatic transmissions rely upon internal clutch and/or brake elements to provide a torque path associated with the selected speed ratio. In a compact transmission with many speeds, the tendency is to minimize the friction area provided by such elements because of space constraints. However, the number of engagements and disengagements will tend to increase as the number of speed ratios is increased, because the speed range covered by each speed ratio is reduced; increased wear of friction material may be a consequence, and it would be desirable to minimize such wear if possible.
Each engagement and disengagement of speed ratio requires expenditure of energy, for example generation of hydraulic pressure to operate one or more hydraulic actuators. A greater number of speed ratios increases the number of engagement and disengagement cycles, and accordingly the energy consumption is also increased. It would be desirable however to minimize such consumption in order to improve fuel economy of a vehicle.
A conventional manual gear transmission generally permits speed ratios to be selected in any sequence; the shift pattern need not be sequential. However a compact automatic transmission having many speed ratios may utilize certain gear wheels in more than one torque path, which may in turn prescribe that speed ratio shifts occur in a predetermined order. The transmission may also include certain physical constraints which require a predetermined shift sequence. Generally speaking the number of prescribed shift patterns or sequences is low compared with the number of potential shift sequences.
It would be desirable to provide a multi-speed automatic gear transmission adapted to rapid downshifting, with minimized wear and reduced energy consumption, whilst accommodating physical constraints which may require predetermined shift sequences.